Coevolutionary relationships between parasites and their hosts can be more complicated than between predators and their prey. As in the case of prey-predator relationship, one can expect hosts to evolve more effective defences against the parasite, and the parasite in turn can evolve to be more virulent. A balancing act has to be done here also as the parasite cannot afford to increase its own reproductive efficiency at the expense of the host. The death of the host would mean the death of the parasite before it could be transmitted to another living host. If the parasite has to be transferred from one host to another the parasite must evolve a lower degree of virulence so as not to kill the host. Successful host-parasite relationships are evolved on the dictum of "live and let live".
The well known Australian example illustrates the coevolution concept between rabbit hosts and myxorna virus parasite. European rabbits Oryctolagus cuniculus were first introduced into the Australian continent in the middle of the 19th century. In less than ten years the population multiplied into enormous numbers occupying a wide variety of environments in Australia from subalpine to subtropical zones. Towards the middle of the 19th century there were several hundred million rabbits. The rabbits had attained the status of a serious pest population. In order to control very effectively the population size, a search of natural enemies for the rabbits was made. Such a search reiealed that myxoma virus could multiply as an epizoite on rabbits and that mosquitoes could be the natural vectors for the mechanical transfer of the parasite.